manual update

doc/MANUAL.md

@@ -93,6 +93,9 @@ preventing 5V to FT231X power pin, but FT231X will still be weakly
 back-powered from its other pins connected with rest of the board 
 and it will appear as some load. For most practical cases, we
 are lucky that FT231X appears as high-z when not directly powered.
+However, unpowered FT231X pins loading may sometimes have prevent
+JTAG from ESP32 or external JTAG, so for more reliable JTAG
+we recommend to keep FT231X powered.
 
 Green LED D18 behaviour is the "Power LED". Green LED ON will keep 
 board powered up. By factory default, when USB-serial chip
@@ -100,7 +103,6 @@ is enumerated by PC, Green LED will turn ON.
 Normally when board is plugged into USB charger Green LED may shortly
 blink and stay OFF, but board will keep being powered.
 
-
 Board PCB v1.7 must be hardware patched to be able to reliably
 enter shutdown mode. (It will keep waking up).